This invention relates to projection display systems which use reflective spatial light modulators, and specifically, to such systems which incorporate reflective liquid crystal devices, as well as to projection display systems which incorporate polarization converters.
1. Projection Display Systems
The system shown in FIG. 1 illustrates the essential components of the optical portion of a projection display system having three reflective spatial light modulators in the form of liquid crystal display (LCD) panels, also referred to as liquid crystal light valves (LCLV). The prior art system, depicted generally at 10 includes a light source 12, an illumination mechanism for collecting the light and concentrating it onto the light valves, shown generally at 14, a polarizing mechanism for polarizing the light, if the light valves modulate via polarization effects, shown generally at 16, a splitting mechanism for splitting the illumination into three color bands to separately illuminate the three light valves, shown generally at 18, a recombining mechanism for recombining the three light distributions after reflecting from the light valves, shown generally at 20, and a projection mechanism for projecting the combined images onto a viewing screen, shown generally at 22.
Lamp 24 and lamp reflector 26 produce and concentrate the light for this system. A series of dichroic filters 28, 30 is used to split the light from the lamp 24 into separate red, green, and blue components. The light in each of the three components, or channels, is then polarized with a polarizing beam splitter (PBS) 32, 34, 36, and illuminates three separate LCDs, 38, 40, 42. The LCDs selectively modify the polarization of the light reflected from them allowing some portion of the light to pass back through the PBS. A second series of dichroic filters, 44, 46, is used to recombine the modulated light distributions and pass them on to a projection lens 48 imaging all three LCDs onto the viewing screen.
The configuration shown in FIG. 1 is functional and has been used to implement projection display system products. However, the large number of components in this architecture is cumbersome, and necessitates a relatively large physical size of the system. The most serious drawback to these systems is the requirement of a large back working distance for the projection lens.
A single filter, or PBS plate, tilted at 45 degrees requires an optical path length equal to or greater than the active width of the LCD panel. It may be seen in FIG. 1 that two of the three channels, green and red, require a PBS and two dichroic filters. These channels require a minimum optical path length between the LCD and the projection lens of three times the active width of the LCD. The blue channel in FIG. 1 requires only one PBS and a single dichroic filter, but the path length must be equal to the other two channels for in-focus registration of all three images on the viewing screen. The actual optical path length for the projection lens must also account for the divergence of the light after reflecting off the LCD panel. This is a function of how fast the optical system is running, usually specified by the f/# of the optical system. The minimum distance referred to here is strictly valid only for systems of very high f/# and thus impractical due to low light throughput. However, for comparison with other systems this minimum figure is a good baseline. The only advantages of this architecture are the ability to optimize the color filtering with the interaction of multiple dichroic structures and the ability to optimize the PBS performance for the narrow band color channels. However, these advantages are relatively minor.
The most straightforward method of simplifying the projector architecture is to have the filter and beam splitter structures perform more than one function in the set of required system operations. System configuration 50, shown in FIG. 2, incorporates two of these simplifications. The first is the use of a single PBS 52 immediately after lamp 24, replacing the three PBS plates of the FIG. 1 system configuration. Single PBS 52 polarizes the broadband output of the lamp prior to the color splitting operation and thus functions as the amplitude modulation control mechanism for all three LCDs. This requires that the PBS function over the entire visible spectrum. The second simplification is to utilize the same set of dichroics to split the light into the three color channels and to recombine the reflected light prior to the projection optic. This requires that the dichroic filter passbands be carefully controlled since there are now only two filters 54, 56, to control the whole system colorimetry. The savings in system complexity is readily evident.
One system difficulty not addressed by the configuration in FIG. 2 is the reduction in the back working distance of the projection lens. The projection lens must still work over a distance that is a minimum of three times the active width of the LCD. A solution to this problem is found by recognizing that the operation of the dichroic filters is still the same even if the dichroic structures are crossed as shown in system 60 in FIG. 3A. This allows the back working distance to be reduced by 33% over the systems of FIG. 1 or 2, to a minimum of twice the active width of the LCD. Unfortunately, crossing plate dichroics 62, 64, introduces a problem because the operation at the intersection of the two plates is usually disrupted by the thickness of the plates, producing a seam in the middle of the image, where the images of the three LCD panels are totally or partially obscured by the plate intersection. Also, the transmission/reflection characteristics of dichroic filters are significantly different for p- and s-polarizations, which results in the color characteristics of the whole projector being difficult to control. This limitation is present in FIGS. 2, 3A and 3B.
The preceding problem is solved in system 70 of FIG. 3B by the introduction of a four piece color cube filter, shown generally at 72. Dichroic filters 74, 76 are deposited on the surfaces of the four cube segments and the pieces are then glued together to form a solid cube with the dichroics sealed in the interior across the cube diagonals. If properly assembled this arrangement eliminates most of the obstruction of the central crossover of the two dichroic layers. However, this assembly is precise and the color cube component is expensive because of the difficulty in assembly. FIG. 3B also shows the use of a polarizing beam splitter cube 78. This component is a common assembly for optical systems and is not an expensive addition due to the significantly less stringent assembly requirements.
As indicated in these system configurations, the state-of-the-art in system architectures for reflective LCDs includes several arrangements, each with particular advantages and disadvantages. A desired alternative is a system that has the small back working distance advantages of the systems shown in FIGS. 3A and 3B, without the costly addition of a precisely assembled crossed dichroic filter cube.
Jacobson et al., U.S. Pat. No. 4,127,322, is fashioned around the optically addressed Hughes liquid crystal light valve (LCLV). A lamp output is polarized by a beam splitter and then divided into the three color paths by dichroic filters. This configuration is equivalent to the system in FIG. 2 of the prior art. The reference also includes an alternative embodiment in which an additional set of dichroic filters and three light valves are arranged to use the light normally discarded by the polarizer. This attempt to recover the unused portion of the light is intended to improve system throughput.
Koda et al., U.S. Pat. No. 4,650,286, and Ledebuhr et al., U.S. Pat. No. 4,836,649, describe architectures for the reflective LCLVs that are essentially equivalent to the system of FIG. 1, with the exception that they use a separate projection lens for each of the three light valves.
Takanashi et al., U.S. Pat. No. 5,239,322, is another system designed originally for the optically addressed LCLV type light modulators. The system covered in this patent is easily recognized as equivalent to the prior art architecture of FIG. 3B. In this system the LCLVs are illuminated with images indicated as write light distributions. CRTs are typically used to produce these write light distributions and are usually abutted directly to the corresponding light modulator.
Ooi et al., U.S. Pat. No. 5,648,860, uses two dichroic plates to separate the light into the three color channels and to recombine the light reflected from the LCDs. The angles of the plates used in this configuration are not 45 degrees and are set to try to reduce the back working distance for the projection optics. One of the primary purposes of Ooi et al. appears to be the use of positive lens elements directly in contact with the LCD panels to collimate the incoming illumination and to converge the reflected light, and the use of “cone-like” prisms to affect the matched convergence of the illuminating light. In all other aspects this system is essentially the same as that of FIG. 2.
Dove, U.S. Pat. No. 5,658,060, is a system having a set of external dichroic filters that separate the light into the three color paths. The light in each path is separately polarized before illuminating the light valves. The reflected light is recombined through a special prism arrangement, usually referred to as a Philips prism. The Philips prism is used to try to reduce the back working distance requirements of the projection lens. Although this system uses a prism for recombination, it is architecturally equivalent to the system in FIG. 1. The reference also describes another embodiment that uses a cube beam splitter for recombining the light output but continues to use separate dichroics to affect the initial split of the light into three color paths and to use separate PBSs for each light valve.
Doany et al., U.S. Pat. No. 5,621,486, describe a simple configuration for a three-panel projector. The system uses a Philips type prism to split the illuminating light and to recombine the reflections from the three LCDs. However, this setup uses a single cube polarizing beam splitter in front of the color splitting prism. This system is thus equivalent to the architecture of FIG. 3.
Sampsell, U.S. Pat. No. 5,233,385, and Poradish et al., U.S. Pat. No. 5,612,753, describe projection systems designed for the TI digital micro- mirror device (DMD) light modulator. These references include system architectures for both single panel color field sequential systems and multiple panel systems. In the multiple panel systems a color splitting prism of the Philips type is used to perform the color separation and recombination and a total internal reflecting (TIR) prism is used to get light on and off of the DMDs. In this case the system looks essentially the same as that of FIG. 3b with a TIR prism used in place of the PBS cube.
References describing transmissive light modulators include Ogawa, U.S. Pat. No. 5,321,448, and Nakayama et al., U.S. Pat. No. 5,626,409, the latter of which describes a system which is the transmissive equivalent of the system in FIG. 1. The light from the lamp is divided into the three-color paths by a set of dichroic filters. After passing through the three light valves, the modulated light distributions are recombined using a separate set of dichroic filters. The '448 reference uses a set of dichroic filters to divide the lamp output into the three-color paths, and a separate set of dichroic filters, in the form of a color cube prism, to recombine the modulated light. This later configuration is the most common architecture presently used for transmissive light valves.
2. Polarization Converters
Projection displays that are based on polarization modulating devices, such as LCDs, include as a necessary step in the LCD illumination process the polarization of the light collected from the system light source. The polarization state required is typically linear with the direction of polarization aligned to a preferred orientation of the light modulator (called the director in an LCD). A typical polarizing arrangement is shown in FIG. 9. Light from the source is passed through a PBS which consists of a pair of prisms assembled to form a cube. A special dielectric coating is deposited on the diagonal of one of these prisms and sealed between the two pieces of glass when the prisms are glued together. The dielectric coating strongly reflects light whose electric field vector is aligned perpendicular to the plane of the drawing in FIG. 9. This light is typically given the designation s-polarized. Light whose electric field vector is parallel to the plane of FIG. 9, typically called p-polarized, is strongly transmitted by the dielectric coating. The light produced by the vast majority of useful light sources is randomly polarized, consisting of an equal combination of s-polarized and p-polarized light. Thus, in a PBS, 50% of the light output reflects as s-polarized and 50% of the light transmits as p-polarized. For a single wavelength of light, it is possible to devise a coating that produces exact polarization separation, i.e., no p-polarization reflects and no s-polarization transmits. However, over the broad visible spectrum, inefficiencies of the coating structure limit the degree of polarization obtainable.
In many projection display systems, light from the system source is polarized to the direction required by the light modulators, and the remaining light, polarized perpendicular to the preferred direction, is simply discarded. This represents a substantial performance reduction since half of the light produced by the source is not useable in the system. A number of different techniques of polarization conversion have been devised in an attempt to recover the performance loss caused by the polarization process. These polarization converters, sometimes called polarization recyclers, employ various techniques to realign the polarization state of the discarded light to be parallel to the desired polarization state for the light modulators, and to add that light back into the illumination system. The setup shown in FIG. 10 implements polarization conversion using only reflection from planar mirror surfaces to accomplish the conversion. The light from a lamp is split into the s and p polarizations by the beam splitter plate (element 302). The s-polarization is oriented in the “horizontal” direction of the perspective drawing. This part of the light distribution is reflected by two mirrors (elements 325 and 326) and remains as an s-polarized distribution when it is incident on the upper part of the illumination region 308. The p-polarized distribution is initially oriented in the “vertical” direction, but reflection from mirror element 323 re-orients this polarization in the “horizontal” plane. A second reflection of this newly s-polarized distribution by mirror element 324 does not effect the direction of polarization and this distribution reaches the lower part of the illumination region 308 as an s-polarized distribution. If we assume a reasonably efficient polarizer with 100:1 polarization ratio, then the setup of FIG. 10 results in approximately 99% of the lamp output incident in the illumination region 308 with the desired, horizontally oriented, s-polarization.
The setup shown in FIG. 10 implements the polarization conversion, but it also illustrates one consideration that must always be taken into account with polarization converters. The illumination region 308 has twice the area of the illumination region that would be covered by either the s-polarized or p-polarized light distributions prior to conversion. One could certainly adjust the mirror elements 324 and 326 to cause the two sub-regions to overlap with the same area as the original lamp output. But, this adjustment would cause the angular distribution of light rays incident on the region to increase. It is a fundamental property of optical systems in general and illumination systems in particular, that, once established, the product of the illumination area and the angular extent of the illuminating rays is an invariant. In an illumination system this invariant is generally referred to as the étendue. In the optical system of FIG. 10, the étendue is established by the lamp, and is dependent on the nature of the reflector and the physical size of the arc source used in the lamp. The process of splitting the two polarization states apart with the beam splitter plate effectively produces a second source as seen by the rest of the optical system. Since the étendue is a geometrical optics property rather than a physical or wave optics property, it does not depend on polarization states. Thus, the effective second source has the same étendue as the original lamp and the total étendue in the rest of the system is twice that of the original lamp. It is clear in FIG. 10 that the net étendue is doubled since the area illuminated is doubled. In the case of adjusting the mirrors to cause the illumination region to have the same area as the original beam, the étendue doubles due to a doubling of the angular extent of the illuminating rays. In either case, the étendue of the output of a polarization converter is twice that of the input randomly polarized input.
A potential problem with this doubling is that the optics after the illumination system may not be able to handle the larger étendue. If the étendue of the rest of the optical system is equal to or greater than that of the polarization converted illumination distribution, then all of the illumination distribution is usable. In this case, the system throughput would be essentially doubled relative to a system that did not employ polarization conversion. If the étendue of the rest of the optical system is smaller than that of the polarization converted illumination distribution, then some of that light will be lost. The loss results from either overfilling the area of the usable field of view in the remaining optical system or by overfilling the numerical aperture of the remaining optics. In this case, the increase in system throughput would be less than the twofold improvement in the fully étendue matched configuration. In the extreme case where the étendue of the rest of the optical system is just matched to the étendue of the original lamp output, there would be no gain in system throughput, since all of the converted light would fall outside the usable area or numerical aperture of that optics.
The polarization converter of FIG. 10 has the advantage of being implemented with very simple components, namely a polarizing beam splitter and front surface mirrors. The disadvantage is the number of components involved—a beam splitter and four mirrors. This can lead to packaging and size problems. A much more common method of implementing polarization conversion requiring fewer components is shown in FIG. 11. In this system, a polarizing beam splitter is again used to separate the s and p polarization states. The s-state light reflected from the beam splitter surface is reflected again by a mirror (or by another polarizer reflecting s-state light) so that its propagation direction is the same as the transmitted p-state light. The s-state light is then passed through a special crystalline optical component called a half waveplate. The half waveplate is made of a material that has different refractive indices along the different directions of its crystalline structure (a characteristic called birefringence). The propagation velocity of light through the material depends on the refractive index. Light polarized in the direction of the crystal axis with the highest index will propagate slower than the light polarized perpendicular to this axis. This axis is called the principle axis of the crystal (sometimes called the extraordinary axis). If linearly polarized light enters the half waveplate with its polarization direction making an angle with respect to the principle axis, then the light can be described as the sum of a component along the principle axis and a component perpendicular to the principle axis. The component parallel to the principle axis is delayed or retarded due to the slower propagation velocity, and this delay causes the net direction of polarization of the light distribution to rotate. In the case of a half waveplate, linearly polarized light making an angle θ with the principle axis on entering the waveplate will be rotated by 2θ upon exiting. If the waveplate in FIG. 11 is set to have its principle axis at 45 degrees relative to the direction of the s-polarized light, then the polarization will rotate by 90 degrees and will be parallel to the p-polarized light transmitted by the beam splitter. The illuminated object now receives only p-polarized light (within the efficiency limits of the beam splitter). It should also be clear from the figure, that the étendue of the illumination distribution at this object is again twice that of the original lamp output due to the larger illumination area.
A slight modification of this setup is shown in FIG. 12 in which the mirror and waveplate are tilted to bring the two illumination patches together. It is clear in this figure that, although the illuminated area is the same as with the original lamp, the angular extent of the illuminating rays has increased. Analysis of this distribution would reveal that the étendue has again doubled.
Another technique for polarization conversion uses a pair of polarizing beam splitters, a quarter waveplate, and a mirror. This method is illustrated in FIG. 13. The unpolarized light from a lamp is passed through one beam splitter where it is again split into its s-polarized and p-polarized components. In the configuration illustrated in FIG. 13, the s-polarized component goes directly to one region of the illuminated object. The p-polarized component of the lamp output passes into a second beam splitter where it again passes through the splitting surface. This light then passes through a quarter waveplate. A quarter waveplate, like the half waveplate described above, is a birefringent crystal that will introduce delays or retardations to the light passing through it. If linearly polarized light enters the waveplate with its direction of polarization at 45 degrees to the principal axis of the crystal, then the output light will be circularly polarized. The “handedness” of the circular polarization depends on the exact relationship between the direction of the waveplate principal axis to the light polarization, but this is not important for the polarization conversion process described here. The circularly polarized output of the waveplate then hits a mirror, where it reflects back through the waveplate. When circularly polarized light reflects from a plane mirror, the handedness (whichever direction it is) is reversed. This reflected light passes back though the quarter waveplate, where it is converted into linearly polarized light by the retardations of the birefringent crystal. However, the handedness of the circularly polarized light passing back through the crystal to the left is reversed from that of the light that exited the waveplate on its first pass. The resulting linear polarization of the light exiting the quarter waveplate to the left will therefore be perpendicular to that of the light entering the waveplate to the right. The p-polarized light that enters the quarter waveplate is thus converted into s-polarized light upon reflection from the quarter waveplate/mirror combination. This light then reflects from the polarizing surface of the second beam splitter and falls on another portion of the illuminated object. As in the previous methods of conversion, the illuminated area doubles relative to the original lamp output, or the light can be redirected to illuminate the same area with a doubling of the angular extent of the illuminating distribution. In either case, the étendue has doubled.
A practical implementation of polarization conversion in conjunction with a fly's-eye integrating illumination system is shown in FIG. 14A and the complete projector system using this illuminator is shown in FIG. 14B. The fly's-eye lens arrays are used to produce a uniform rectangular illumination distribution at the LCLV. An array of polarizing beam splitters is introduced into this setup where the first lens array brings light to focus. As can be seen in the enlargement in FIG. 14A, the PBS array consists of polarizing beam splitters and half waveplates to convert essentially all of the light to s-polarization. This conversion process is identical to that implemented by the setup in FIG. 10.
Another type of polarization converter is disclosed in Heynderickx, et al. U.S. Pat. No. 5,626,408. This patent describes a system in which cholesteric filters are used to split up the output of a lamp into the red, green, and blue color channels needed for a three-panel LCD projector. A polarization conversion technique appropriate for cholesteric filters is implemented in this system. Referring now to FIG. 1 of that patent, light from a lamp 3 is directed into an illumination system. The first element of that system is a cholesteric filter 9. Color filter 9 reflects red light with right handed circular polarization and transmits all the rest of the lamp output. The reflected red light is directed to a mirror 15 where it reflects directly back to the cholesteric filter. The reflection of the light at the mirror changes it into left handed circular polarization and this light passes through the cholesteric filter toward element 63. The next cholesteric filter 10 reflects red light with left handed circular polarization and directs that light toward element 63. All of the red light from the lamp output has now been converted into left handed circularly polarized light. Element 63 is a quarter waveplate that converts this light into linear polarization that is required by the LCD modulator 27. The blue and green portions of the lamp output are converted into left handed circular polarization in an identical process using cholesteric filters 11 and 12 for green and filters 13 and 14 for blue. Quarter waveplates 65 and 67 convert the circularly polarized distributions into the linear polarizations needed by the LCD modulators 29 and 31.
Heynderickx, et al., also disclose a composite circular polarizer for converting unpolarized light into two polarized beams. The polarized beams are then reflected by cholesteric filters toward LCD modulators.
Takanashi et al., U.S. Pat. No. 5,122,895, describe a polarization converter using a pair of cube polarizing beam splitter and a quarter waveplate/mirror component identical to that of FIG. 13. A key feature of this invention is that the quarter waveplate is made an active component by implementing it with a liquid crystal or other electro-active optical material. This allows the conversion process to be switched on and off with a control signal.
Takanashi, et al., U.S. Pat. No. 5,164,854, describe several implementations of polarization converters for a reflective LCD projection system. The implementations include a beam splitter half waveplate configuration essentially identical to that of FIG. 11 and a non-switchable quarter waveplate implementation essentially identical to that of FIG. 13. In all cases in this patent effort was made to realign components to direct the converted polarized light to a small illumination area with the corresponding increase in angular distribution of the light.
Karasawa, et al., U.S. Pat. No. 5,200,843 and Karasawa, et al., U.S. Pat. No. 5,278,680 describe implementations of all three types of converters shown in FIGS. 10–12.
Nicolas et al., U.S. Pat. No. 5,299,036, describe a projector that uses two full color LCD modulators. A beam splitter divides the light to the two modulators and one path has a half waveplate to rotate the polarization to be properly aligned with the LCD director. The rest of the optical system is involved with recombining and aligning the two projected LCD panels. Although this uses two separate modulators the concept is similar to the beam splitter/half waveplate conversion method of FIG. 11.
Blanchard, et al., U.S. Pat. No. 5,303,083, describe a polarization converter that has features of both the beam splitter/half waveplate configuration of FIG. 11 and the beam splitter/quarter waveplate configuration of FIG. 13. The light normally lost from a polarizing beam splitter is first converted into circular polarization by a quarter waveplate, reflected by a 45 degree mirror, and then passed through a second quarter waveplate to produce linear polarization in the same direction as the desired output from the beam splitter.
Shingaki et al., U.S. Pat. No. 5,381,278, and Mitsutake, et al., U.S. Pat. No. 5,566,367, describe a number of polarization conversion techniques that are essentially equivalent to the beam splitter/half waveplate method and the beam splitter/quarter waveplate method described in the previous section. These patents also describe arrays of small conversion prism structures that are used in conjunction with fly's-eye integrators in a projector illumination system. The configuration of the array of converters is similar to that of the system shown in FIG. 14.
Kato, U.S. Pat. No. 5,653,520, describes a large multi-segment converter using beam splitters and half waveplates. The configuration is intended for use with large LCD panels and its multiple segments are used to achieve full conversion and uniform illumination over a large light modulator. The conversion process is identical to that of the beam splitter/half waveplate process.
Miyatake et al., U.S. Pat. No. 5,657,160, describe a method of polarization conversion that is affected purely by reflections from plane mirrors without using waveplates. The process is essentially equivalent to the process described in FIG. 10. The patent also describes an array of small converters using this process that employs internal reflections inside small prism structures. This structure is also used in conjunction with fly's-eye integrators in a projector illumination system.
All of the aforementioned polarization converters described above convert all of the light output of a lamp, in all three-color bands, into a single linear polarization state. Heynderickx et al., U.S. Pat. No. 5,626,408, mentioned as the exception above is the only patent that describes treating colors separately, but this was done only to implement the color splitting with cholesteric filters. The system described in that patent ultimately put all three-color bands into the same linear polarization state.
Accordingly, there is still a need for a projection system for reflective liquid crystal light valves that has a small back working distance for the projection lens, that has a projection path free from crossed dichroics obstructions, that has a relatively simple configuration of the components that are used to split up and recombine the three color light distributions, includes an efficient polarization converter for higher brightness applications, that is more compact than known projection systems, and that may be manufactured at a reduced cost compared to existing systems.